Aircraft Structures

In this tutorial, you will learn the basics of aircrafts structures which includes its definition, station numbers, classifications, and about different materials used in aircraft structures.

Contents:

1. Aircraft Structural Design
2. Aircraft Station Numbers
3. Nomenclature and Definition
4. Cockpit, Cabins, and Compartments
5. Tail and Control Surfaces
6. Landing Gear
7. Powerplant Structures
8. Rotorcraft Structures

Aircraft Structural Design

The structural design of the aircraft is one of the most important aspect which supports the airplane during extreme conditions.

• The aircraft must withstand huge amount of load than, that imposed by its own weight. It is designed to withstand one and one-half times the maximum expected loads. These loads imposed on it is stated in terms of Load Factor.
• Load factor is the ratio of the total load of the airplane’s wing to the actual weight of the airplane. All airplanes are designed based on certain criteria’s such has strength and the type of use of that airplane.
• Based on strength and operational use of the airplane, it is classified into three category system:
• Normal Category
• Utility Category
• Acrobatic Category
• The airplanes that have load factor limit of 3.8 is of normal category. The utility category is used for limited acrobatic operations with a load factor limit of 4.4. Acrobatic category is free to operate and it does not have any kind of restrictions unlike normal and utility, it has a load factor limit of 6.0.

The below diagram shows the classification based on strength and operational use.

The above diagram shows the classification based on strength and operational use, by which the airplanes are certified for its particular operational purposes along with its load factor limit.

Aircraft Station Numbers

For service, maintenance, and repair of the aircraft, there is certain points of reference and station numbers for the fuselages, wings, nacelles, empennage, and landing gear.

• The ATA has set certain zoning specifications in ATA-100 specification for manufacturers technical data. In fuselage stations, there are longitudinal points along the fuselage of an airplane determined based on the datum line from zero.
• Datum line is located in the forward edge of the wind shield of the airplane, in every inches forward and aft the station numbers are given in the datum line. The station numbers in the forward parts are negative, and the station numbers aft of the datum line are positive.
• To locate reference points on the wing surface, the wing section is measured from the center line of the fuselage. Hence, this line is called as butt line. In right or left of the fuselage centerline, the wing stations are represented in every inches.
• For locating stations in the vertical line, a line is established and it is called as water line. The waterline is originated from the design and building of ship hulls. It was also being used as a vertical reference.

Nomenclature and Definition

The below mentioned definitions are based on the structural components of aircraft.

• Aileron: It is hinged section of the tailing edge of the wing section which provides lateral control.
• Airfoil: It is a surface that provides reactions with respect to the movement obtained in air.
• Cantilever: It is a member which has one supported end without any external bracing.
• Cockpit: An area occupied by the pilot is called as cockpit.
• Cowling: A detachable cover which can be removed and placed back on, especially an engine.
• Elevator: It is a hinged section of the horizontal stabilizer which usually helps to decrease and increase the angle of attack of the airplane.
• Empennage: It is a surface to which the rudder and the elevators are attached.
• Span: The maximum distance that is measured parallel to the lateral axis of the airplane.
• Stabilizer: The tail surface that is fixed horizontally to maintain stability around the lateral axis of the airplane.
• Stringer: Longitudinal member in the fuselage that transmits skin loads into the body frames or wing ribs.
• Vertical fin: It is referred to as vertical stabilizer because it provides directional stability and the rudder is formed when the trailing edge is hinged to it.
• Water line: A horizontal reference line that is parallel to the ground and used to locate points vertically.
• Frame: A circumferential structural member that provides support for the stringers and the skin.
• Fairing: A streamlined contour structure that is used to cover non-streamlined objects.
• Cowl panel: The removable sides which is hinged to the sides of the pods and the nacelles, it is mainly used to cover the engine.

Cockpits, Cabins, and Compartments

The cockpit, cabins, and compartments are the internal components of the fuselage where all the major functioning takes place.

• The fight crew occupies the major portion called as the cockpit. The controls that are used to fly an airplane is all operated only from the cockpit. The word ‘control’ means that the pilot operates certain functioning like speed, direction of flight, attitude and the power of an aircraft.
• While designing the aircraft, the engineers ensure to provide proper headroom, visibility, clearance for controls, and some space for movement. The space enclosed for the flight crew is called as cabin, flight deck or the crew compartment.
• When flying through rain or snow, the structure must be waterproof so that no water leakage occurs inside the crew compartment. By design and proper insulation, there should not be any vibration or noises within the cockpit.
• Generally, in commercial aircrafts lockable door must be present in between the cockpit and the passengers. The passenger compartments should have maximum comfort and safety which is very important.
• The aircraft fuselage is designed with adequate openings for doors, windows and windshields. These openings are especially used for certain operational purposes for the loads to flow through it. The metal used is an aluminium alloy and the door frame is made up of hydropress, stamp press or drop hammer.
• The cargo compartments are designed to maintain all the requirements in order to carry the load under all normal conditions of flight and landing without failure of any structural component. The transport type aircrafts are specially designed to carry heavy loads which enables them to carry heavy freight in addition to baggage for the passengers.
• A combi aircraft is a combination where it carries both the combination of passengers and cargo. These are separated by some temporary or movable bulkheads. Depending upon the demand, these types of aircrafts can be configured for either increased passenger or cargo loads respectively.

Tail and Control Surfaces

The tail and the control surfaces include the horizontal stabilizer, vertical stabilizer and the control surfaces.

• The stabilizer and the control surfaces are constructed same as that of the wing, but in a smaller scale and it usually contains one or more spars and ribs attached to it. The vertical stabilizer is constructed in multiple ways either as a part of the fuselage or may be separate member, which can be both adjustable and removable.
• The tail section of the fuselage is formed by stabilizers, elevators, and rudder and this is commonly called as empennage. To provide longitudinal pitch stability, the horizontal stabilizer is attached to the aft portion of the fuselage structure.
• The location of the horizontal stabilizer varies, it can be either above or below the vertical stabilizer. Sometimes, it may be in the midpoint of the vertical stabilizer and it is generally constructed using wood, steel tubing, sheet metal, or composite materials.
• The rear spars are little heavier than the front spars. The large loads are imposed on the stabilizers from the attached rudder and elevators, so it requires the front spars to be heavier than the rear spars.
• To provide pitch trim, the stabilizer is normally attached to the fuselage with a pivoting hinge. As its rear spar and the front spar has an attachment for a mechanical or a hydraulic actuator, which is controlled by the pilot to move the leading edge of the stabilizer in order to change the trim of the aircraft.
• The airfoil section that is forward of the rudder is the vertical stabilizer of the airplane and it provides the longitudinal stability (yaw) for the aircraft. Hence this unit is commonly called as the fin. The construction of the vertical stabilizer is similar to that of the horizontal stabilizer.
• Dorsal fin is attached in many of the aircrafts and its major function is to improve the longitudinal stability of the aircraft. It also provides a streamline fairing between the vertical stabilizer and the fuselage.

Landing Gear

The landing gear is one of the most important construction and the method of attachment. It is considered because of transmitting the landing loads to the aircraft without overstressing on the aircraft structure.

• Generally landing gear is classified as either retractable or fixed. It can also be classified according to the arrangement of the aircraft. A fixed landing gear holds the airframe in a fixed position. This type of landing gear generates moderate amount of aerodynamic drag.
• In case of retractable landing gear, is carried into the airframe structure to reduce drag. While landing takes place, the landing gear is extended and the gear is retracted into the airframe after the take-off is completed.
• The aircraft can also have a combination of both the type of landing gear to serve special purposes. The conventional and the tricycle arrangements are the most commonly used configuration. There are two main landing gears in both conventional and tricycle arrangements.

Powerplant Structures

The powerplant structure installed inside the aircraft is the main source of thrust production during the aircraft’s cruising, landing and take-off.

• The engines inside the aircraft are responsible for the vibration, heat, drag force, possible fire, and concentrated thrust. It is usually enclosed by a housing called a nacelle and it is attached using an engine mount to the aircraft. The engine is isolated from the aircraft by a barrier called as the firewall.
• The engine nacelle provides the support and protects the components of the aircraft, its main purpose is to serve as a covering to the aircraft’s engine and also it helps to improve the aerodynamics of the aircraft. The nacelle directs the airflow in order to maintain the engine temperature while combustion and helps in proper exhaust outflow.
• The nacelle has removable parts in order to access engine effectively for maintenance purposes. In a reciprocating engine the cowl flaps incorporated in the nacelle controls the engine cooling. The nacelle also includes several engine monitoring systems such as fire detection and thrust indication.
• The engines and the auxiliary power units must be isolated by means of firewalls, shrouds and other equivalent means in order to protect the nacelle strut from exposure of extreme conditions. All these are sealed with close-fitting fireproof grommets and bushings.

Rotorcraft Structures

The aircraft components used on airplanes is common to the helicopter and it has rotor system that is driven by engine, that geneThe evolution in constructing the helicopter structures is by using aluminium semi-monocoque and monocoque designs. The material that has higher strength to weight ratio and has better aerodynamic quality are the sheet metals. Another type of construction is being incorporated that is combination of materials like graphite, kevlar, and fiberglass that is combined with bonded structures.rates adequate amount of lift and propulsive thrust.

• In helicopters, there is only one main rotor and the others rely on an auxiliary rotor in order to overcome the torque that is produced by the main rotor and this maintains the directional control. In a fixed wing aircraft, the lift forces are concentrated on the wing and the thrust force is being centered on to the powerplants.
• The helicopters usually have both the lift and the thrust forces concentrated on the main rotor. It experiences high vibration due to the rotating components in it, this eventually contributes to metal fatigue. The materials that are used in fixed wing aircraft and rotorcraft was similar, but now the helicopters use different materials that is used for construction.
• The changes in materials are only because of problems with fatigue, vibration, and weight. So, in order to resolve these problems, the manufactures have adopted composite materials for the construction of helicopters. The tubular construction has poor strength to weight ratio, but it is strong and it also has poor aerodynamic qualities.
• The evolution in constructing the helicopter structures is by using aluminium semi-monocoque and monocoque designs. The material that has higher strength to weight ratio and has better aerodynamic quality are the sheet metals. Another type of construction is being incorporated that is combination of materials like graphite, kevlar, and fiberglass that is combined with bonded structures.